In the sentence 'The rational is rewarded'. Why is rational an adjective in the sentence.

Because the word "rational" takes on the function of modifying the noun, in addition to representing a characteristic.

Explanation:

Adjectives are words that accompany a noun and modify it, adding a characteristic that particularizes that noun. In this case, we can say that in the sentence above, the word "rational" is the adjective, since it specifies what is "rewardable".

It should be noted that adjectives accompany nouns in number, gender and degree.